The present invention relates to the electrodes of a single ended arc discharge tube. Such tubes are typically used in conjunction with a reflector for low wattage task lighting applications and for high wattage stage/studio lighting applications.
In single ended arc discharge tubes, it is desirable that the arc occur at the extreme free ends of the electrodes and in the center of the envelope of the arc tube. Conventional designs include the linear electrodes disclosed for example in the Keeffe, et al., U.S. Pat. No. 4,321,504 dated Mar. 23, 1982 and in FIG. 1 of the appended drawings. Electrode breakdown deposits tend to accumulate at the coolest spot in the bulb, generally the point where the electrodes emerge from the pinch seal forming a conductive layer, and thus tend to reduce the distance between the electrodes and cause arcing at points other than the free ends. The location of the arc near the pinch seal not only moves the arc from the desired location in the center of the arc tube envelope, but results in rapid deterioration and failure of the lamp.
In an attempt to avoid the "Jacobs Ladder" effect of one or more arcs intermediate the length of the electrodes, the prior art has reduced the spacing between the free ends of the electrodes related to the spacing of the electrodes elsewhere within the arc tube envelope. The electrodes may be curved towards each other such as illustrated in the Rothwell, Jr., et al., U.S. Pat. No. 4,415,829 dated Nov. 15, 1983 and shown in FIG. 2 of the drawing appended hereto. In other prior art designs, the electrodes may be angled towards each other as shown in the Keeffe, et al., U.S. Pat. No. 4,308,483 dated Dec. 29, 1981 and illustrated in FIG. 3 in the drawing appended hereto. In still other prior art designs, one of the electrodes may be angled and the other curved to provide a generally axial arc. By way of example, such electrode structure is shown in the English, et al., U.S. Pat. No. 4,454,450 dated June 12, 1984 and is illustrated in FIG. 4 of the drawing appended to this application.
The attempts of the prior art to deal with this undesirable arcing problem have not been entirely successful. The spacing between the ribbons or foils which connect the lead-in wires to the electrodes is generally limited by the pinch seal, and is typically well inside the inside walls of the arc tube envelope. This spacing has dictated the maximum spacing of the electrodes at the point where they exit the pinch seal and enter the envelope of the arc tube. The inclination of the electrodes towards each other from that point tends to reduce the undesirable arcing problem short of the extreme free ends, but has not been fully effective because of the limited spacing available at the egress of the electrodes from the pinch seal. To increase the space between the electrodes, it is known in some lamps to increase the width of the pinch. However, such an increase in width often affects undesirably other operating characteristics of the lamp.
It is accordingly an object of the present invention to obviate such problems and to provide a novel single ended arc discharge tube.
Another object of the present invention is to provide a single ended arc discharge tube in which the electrodes enter the envelope of the tube at a spacing greater than that of the ribbons connecting the electrodes to the lead-in wires.
Still another object of the present invention is to provide a novel single ended arc discharge tube in which the electrodes are generally concave towards each other between the ribbons and the free ends thereof.
Another object of the present invention is to provide a novel single ended arc discharge tube in which the electrodes diverge within the pinch seal of the arc tube.
These and other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims and the following detailed description when read with the appended drawings.